The present invention relates to a thermal protection device for electrical conductors, and more particularly, for the electrical conductors of industrial electroheat, inductive, arc, resistance or radiation furnaces.
The conducting turns of the large inductors of industrial induction furnaces are generally formed as hollow copper, profiled tubes. The copper tubes are cooled by circulating water which removes the ohmic losses and the heat released by the high temperature inductive heating load. The profiled tubes are further covered with a layer of electrical insulation, which maintains its mechanical and dielectric properties only up to temperatures of arounnd 200 to 220xc2x0 C., or perhaps 300xc2x0 C. in the case of the highest-performance insulations.
This electrical insulation is frequently damaged or burnt since the conducting turns are exposed to various potentials for damage, for example, to splashes of molten metal, to very high levels of heat radiation coming from the crucible or from other heated components (heated white hot), or to streams of gases (carbon, hydrochloric acid, hydrogen, etc.) at high temperature in pyrolysis furnaces.
A defect in the electrical insulation can generate inter-turn arcing, arcing between the turns and the structure of the grounded inductor, or between the turns and the grounded load.
A known solution for remedying this problem is to embed the inductor in a cylinder of refractory concrete or cement. This solution applies to induction heaters for billets, but cannot be generalized since it has several drawbacks. For example, such embedded inductors are bulky and heavy. The refractory coating is cast on site, and replacing an inductor in a large furnace becomes very lengthy and expensive. In the event of a minor fault in an inductor (electrical fault, water leak, etc.), it is very difficult to locate the fault, in order to carry out a partial repair, without having to break the entire concrete encapsulation and without damaging the sound electrical insulation underneath. Also, such embedded inductors are difficult to install and to maintain.
The objective of the present invention is to find a thermal insulation which is capable of withstanding heat shocks and which can solve the above-mentioned problems. Such a solution must, in particular, be lightweight, easy to install, inexpensive and dense enough to be impermeable to heat radiation and to any infiltrations, for example, by carbon or pitch particles.
The thermal protection of the present invention will not be limited to industrial furnaces, but can also protect the electrical conductors of machines that have to operate as long as possible in the event of a fire, such as motor-driven smoke-exhausting fans, safety lighting, etc.
The foregoing objective is achieved in accordance with the present invention by a thermal protection device for an electrical conductor covered with an electrically insulating layer which is required to conduct, for variable durations, a current under high external temperature conditions which are potentially damaging to the electrically insulating layer. The thermal protection device is formed from at least one layer of a composite refractory produced by a combination of refractory fibers and a compatible refractory binder, so as to constitute a heat shield which is able to withstand external temperatures of between 500 and 1,600xc2x0 C.
The refractory fibers can be in the form of woven or nonwoven tapes, or in the form of a paste. When implemented in the form of tapes, the tapes are arranged in at least one thickness of overlapping windings around the inductor.
The resulting heat shield can also include a number of refractory tiles tangentially cemented or adhesively bonded to the tubes. Prior taping of the conductor with refractory fibers permits good attachment of the refractory adhesive to the electrically insulated conductor.
The present invention will be more clearly understood with reference to the description given below, together with the following figures.